FIG. 1 shows a general tape dispenser 1, which includes a tape holder 11 for supporting a tape roll (not shown) thereon, a blade 12 located in front of the tape holder 11, and a roller 13 located below the blade 12. The tape may be pulled from the tape holder 11 to the roller 13 for bonding to something, and then cut by the blade 12 when the bonding is completed. Tapes of different adhesion ability are available. When a tape with relatively high adhesion ability is mounted on the tape dispenser 1 for use, the tape holder 11 must have a high immobility, so that the tape could be successfully pulled out from the tape holder 11 and be cut by the blade 12. For this purpose, there is a tightening mechanism provided on the tape holder 11 of the tape dispenser 1. FIG. 2 is an exploded perspective view of the conventional tightening mechanism for this purpose. As shown, the tape holder 11 of the tape dispenser 1 is provided at a central portion with an axial hole, an outer end of which is formed into an expanded round recess 111 having a straight peripheral wall for a washer 14 and a locating member 15 to fit therein. The tape holder 11 is mounted on a fixing bar 16, and a turning knob 18 having a spring 17 put thereon is sequentially extended through the locating member 15, the washer 14, and the round recess 111 to screw to the fixing bar 16. When the turning knob 18 is turned forward, the washer 14 is forced against the round recess 111 to enhance an immobility of the tape holder 11. In the above-described conventional tightening mechanism, since the force generated by the turning knob 18 is vertically transmitted from the locating member 15 to the washer 14 and the round recess 111, the tightness of the washer 14 against the round recess 111 is not adjustable. Therefore, the conventional tightening mechanism is not ideal for use, and the immobility of the tape holder 11 is not adjustable according to the adhesion ability of the tape being used on the tape dispenser 1.